Conventional communications systems typically require an interface system to couple a signal from an output device with low characteristic output impedance (e.g., about 8 to about 500 ohms) to a receiver while producing minimal signal distortion. The low impedance output may be the output of another device, such as a gigabit interface converter (GBIC), or the output of a fixed impedance transmission line, or a serializer/deserializer (SERDES), or the like. Data signals in digital communication systems are typically square waves having at least two voltage levels (e.g. 0V and 5V), transition times between the two levels (i.e. rise time, fall time), and a period that is related to the pulse width. The shorter the period and faster the transition times, the higher the data rate and the frequency of the signal. The interface system commonly provides the necessary signal modifications to the electrical characteristics of the output signal to match the input requirements of the receiver. For example, the maximum input voltage for a certain receiver may be lower than the voltage of the signal of a corresponding output device. Hence, the interface system must attenuate the signal prior to reaching the input terminals of the receiver.
Another common interface mismatch is the DC voltage component of the signal, commonly referred to as the DC offset. For example, a signal traveling through a coaxial cable is typically an AC signal referenced to the static ground or DC voltage level of the outer conductor of the coaxial cable. However, a receiver may require an input signal with a certain DC offset to provide a DC bias voltage and prevent saturation of its input state. Therefore, to overcome DC offset mismatch problems, AC or capacitive coupling interface systems are desired over direct or DC coupling systems.
Some AC coupling interface systems employ a resistive-capacitive network (RC) to couple fixed impedance output devices with receivers. The capacitive and resistive elements of the network are chosen to transmit the signal without distorting the information carried by the signal. In addition, selection of the elements should take into account the modifications to the electrical characteristics of the signal required to match the receiver input requirements. The product of the resistance times the capacitance of the RC network elements provides the RC time constant (TC) that must be selected in relation to the period of the data signal to avoid signal distortion.
Another factor to consider in the design of AC coupling interface systems is the energy transfer between the output device and the receiver. In order to transfer the maximum signal energy through the interface system from the output device to the receiver, the interface system should provide proper impedance matching between the characteristic output impedance of the output device and input impedance of the receiver. For example, in an RC network interface system, the resistive element is chosen to match the characteristic impedance of the output device, e.g. a transmission line, a GBIC, or the like. Proper impedance matching, or termination prevents signal reflections at the interface that may cause distortions (i.e. ringing). Hence, to provide proper matching to a fixed impedance device, the resistive elements of an RC network may be predetermined by the characteristic impedance of the output device. Similarly, the capacitive elements may be determined by the period of the signal, as described above, the capacitive value must be selected to provide a TC that does not cause signal distortion.
A problem arises when the data signals supplied to the receiver recur at data rates that are orders of magnitude apart, the characteristic output impedance of the output device is very low (tens to hundreds of ohms), and the receiver is configured to receive multiple standard digital data rates. For example, a high-speed signal output from an optical GBIC terminal at 2.5 Gbps and a lower-speed RS 232 signal at 9.6 Kbps may be supplied at different times to the input of a wide-band receiver through the same low impedance transmission line. In this situation, an AC coupling interface system must provide matching for the low impedance of the output device while preserving the signal data integrity, i.e. not distorting the signal. Such an interface system including an RC network and operating the very low characteristic output impedance (e.g. 50 to 100 ohms) would require an impractical, prohibitively high value capacitive element to maintain a reasonable RC time constant that does not distort the lower frequency signal beyond practical use.